The present invention relates to voltage and electric field measuring devices utilizing optical polarization. More particularly, the invention relates to a voltage and electric field measuring device of which the temperature characteristic is improved.
Fundamentally, a measuring device of the general type to which the invention pertains operates by measuring a voltage applied to an electro-optic crystal. Such a measuring device can be used as a voltage measuring device if electrodes connected to both surfaces of the electro-optic crystal are connected directly to terminals across which a voltage to be measured is applied and may be used as an electric field measuring device if it is placed in an electric field to be measured with the electrodes removed.
An example of a voltage measuring device utilizing light is shown in FIG. 1A. A polarizer 2, an electro-optic crystal 3, a quarter-wave plate 4 and an analyzer 5 are arranged in the stated order in the direction of advancement of light from a light source 1. A voltage to be measured generated by a voltage source 6 is applied to the electro-optic crystal 3.
The polarizer 2 converts the light from the light source 1 into linearly polarized light and the electro-optic crystal 3 subjects the linearly polarized light to a phase change resulting in elliptically polarized light. When the voltage to be measured, hereinafter referred to as the "measurement voltage" when applicable, is at zero, the electro-optic crystal has refractive indices n.sub.x and n.sub.y and when the measurement voltage is at V volts, the refractive indices are changed to n.sub.x -kV and n.sub.x -kV where the refractive indices n.sub.x and n.sub.y are those respectively for linear polarization in the x direction and for linear polarization in the y direction and k is a constant. If linear polarization in a direction x.sub.1 is split into vector components in the x and y direction, the refractive indices in the x and y directions are different so that the speed of the light is different for the two directions. Because of this, the linearly polarized light is converted into elliptically polarized light due to the phase difference between the x and y direction components. The analyzer 5, which is disposed in a cross Nicol position with respect to the polarizer 2, changes the amplitude of the elliptically polarized light.
With the power of light incident on the polarizer 2 is represented by P.sub.in and the amount of loss at the measurement section is represented by l, the relation between the light output power P.sub.out and a voltage V.sub.in to be measured can be expressed by the following equation (1) when the quarter-wave plate 4 is absent: ##EQU1## where V.sub..pi. is the half-wave voltage which depends on the type of crystal used and its orientation in use.
It is desirable to operate upon a nearly linear portion of the characteristic curve of the equation (1). For this purpose, it is necessary to shift the operation to the point .lambda./4 as shown in FIG. 1B. To accomplish this, the quarter-wave plate 4 is provided which serves as optical biasing means. When the quarter-wave plate 4 is inserted, the following equation can be obtained from modifying the equation (1): ##EQU2##
In the range of ##EQU3## the equation (1) can be rewritten as the following equation (2): ##EQU4## The significance of equation (2) is illustrated in FIG. 2. The output optical signal from the analyzer is converted into an electrical signal by an element such as a PIN photodiode.
The voltage measurement is carried out according to the above-described principles. For voltage measurement, crystals of KDP, ADP, LiNbO.sub.3 and LiTaO.sub.3 can be used for the electro-optic crystal 3. However, the use of these crystals is disadvantageous in that the measuring device then has an unsatisfactory temperature characteristic because the refractive indices n.sub.x and n.sub.y are somewhat different and they generally have different temperature characteristics. In other words, such crystals have a natural birefringence with, for instance n.sub.x =n.sub.e -kV and n.sub.y =n.sub.o +kV each of which has a different temperature characteristic. This can be understood from the graphs of FIGS. 3A-3C which show examples of the temperature dependence of ordinary rays and extraordinary rays.
In order to compensate for this temperature instability, a so-called "temperature compensation type" has been proposed in which two crystals A and B are coupled together with their axes oriented in different directions. With this type, the light passing through the analyzer 5 can be described by the following equation (3): ##EQU5## where .psi..sub.o ' is the polarization angle for ordinary light, .psi..sub.e ' is the polarization angle for extraordinary light, and .gamma..sub.c is the Pockel's constant. If the crystals are precisely machined so that the length l.sub.1 of the crystal A is equal to the length l.sub.2 of the crystal B, the temperature dependence of the term including the difference between the refractive index n.sub.o of an ordinary ray and the refractive index n.sub.e of an extraordinary ray can be theoretically eliminated.
However, in practice, it is considerably difficult to precisely machine the crystals so that the lengths l.sub.1 and l.sub.2 are precisely equal, to couple the crystals together, and to mount the crystals thus coupled in a casing without imparting stress to the crystals. Thus, it is extremely difficult to manufacture such a temperature compensation type device.
FIG. 5 shows a temperature characteristic curve of a temperature compensation type voltage measuring device using a LiNbO.sub.3 crystal. In FIG. 5, relative values plotted on the vertical axis are output voltage/average received light powers, namely, ##EQU6## from the equation (2).
In view of the above-described difficulties accompanying a conventional voltage and electric field measuring device, an object of the present invention is to provide a voltage and electric field measuring device in which the electro-optic crystal has a high temperature stability and which can be easily manufactured.